1. Field of the Invention
This invention relates to an optical apparatus arranged to decide on the basis of an object image a focal length to be changed.
2. Description of the Related Art
A so-called automatic zoom device or a camera of the kind arranged to keep a photographic magnification unchanged or to automatically set a focal length has heretofore necessitated setting beforehand, by some means, the photographic magnification to be kept unchanged or information related to the photographic magnification. It has been arranged, in some cases, to obtain the desired photographic magnification by first detecting an object distance and then by deciding a focal length according to the object distance. Such an arrangement has been disclosed, for example, in U.S. Pat. No. 5,173,807.
In the conventional arrangement mentioned above, however, a focal length is decided solely according to the object distance. Therefore, it has been impossible to obtain an angle of view apposite to the object of shooting unless the object is an object anticipated or the size of the object is approximately the same as that of the object anticipated.